Method and apparatus for sewage water treatment

ABSTRACT

Method for sewage water treatment using suspended activated sludge where sewage water is supplied to the equalizing tank being then re-pumped to the activation tank from which is supplied to the post-sedimentation tank after the clearing process and from here, after the remaining sludge sedimentation, to the drainage. The activation process is automatically interrupted after a drop of the sewage water level in the equalizing tank below the minimum level setting and the excessive sludge is then pumped off from the activation tank. The sludge re-pumping is interrupted and the activation process restored owing to the subsequent raising of the sewage water level in excess of the operation level setting.

TECHNICAL FIELD

The invention relates to the method and apparatus for a sewage watertreatment using the biological activating system with suspendedactivated sludge in particular for small domestic sewage water treatmentplants.

BACKGROUND ART

The biological method of-the sewage water treatment consists inutilizing the activated sludge formed by a mixture of various bacteriaand small microorganisms. For its existence, this sludge needs organicsubstances contained in sewage water which is decomposited and thuscleared by these substances. The activating process is possible only dueto a continuous oxidation introduced as a rule by blowing air into theactivation tank.

For the sewage water treatment are used partly microorganisms seatedfirmly on their base in the form of various systems of biologicalfilters and reactors wetted by sewage water, partly activating systemswith suspended sludge where the sludge floccules are mixed together withsewage water and with air.

The sewage water treatment plants using the suspended sludge method knowhirherto can be devided into systems using a continuous sewage waterpassage through the activation tank, and to systems using adiscontinuous or intermittent passage.

In the continuous sewage water treatment systems, the sewage water issupplied after its coarse pre-treatment to the activation tank and,after a technologically necessitated period needed to its clearing,drained to a separated post-sedimentation tank together with theactivated sludge. In this tank, the sludge is finally separated bysedimentation on the cleared water being drained away.

In the system using the discontinuous sewage water passage, the sewagewater is supplied after its coarse pre-treatment to the activation tankeither immediately or having been re-pumped from the equalizing tank.After the water treatment, the activation process is interrupted, i.e.the aeration and the water mixing, if any in the activation tank arestopped the cleared water being pumped or drawn by gravity to thedrainage after the sludge sedimentation. After this, the activation tankis refilled the above described treatment cycle being repeated. Comparedwith the continuous sewage water treatment method, thepost-sedimentation tanks can be omitted the activation one (SBR) beingrefilled in cycles.

The disadvantage of the above described sewage water activationtreatment systems is their difficult utilization for small householdtreatment plants respecting in particular the demands connected with thetreatment plant operation control.

In the activation treatment plants using the continuous sewage waterpassage, the sludge must be continuously re-pumped from thepost-sedimentation tank to the sewage water inflow to the activationtank, As soon as the sludge concentration in this tank exceeds thepermitted value, the excessive sludge must be pumped away from thesewage water treatment plant, A skilled operator is required to performregular measuremants of the sludge concentration in the activation tankand to remove the sludge. In addition to this, a low sewage water inflowwould cause an intermittent load of the activation tank. This would befollowed by a deteriorated drainage water quality or by a necessity ofoverdimensioning the activation and the post-sedimentation tanks toobtain the required drainage water parameters.

With respect to a sludge accumulation in the activation tank, thehitherto known small sewage water treatment plants with a continuouswater passage are designed either for a high sludge density where thesludge suspension must be maintained using a method with high demands onenergy for up to 200 days till the sludge drainage without anyinterruptions of the water treatment plant function, or they requireskilled operators draining regularly the sludge away from the activationtank. Both systems cannot be kept functioning for a longer time periodwithout sewage water inflow due to the sludge autolysis followed by thesludge remowal from the activation process introduced stepwise due tothe consumption of activation substances in the activation tank. In thisway, the water treatment plant function is substantially affected. Theactivation sewage water treatment plants with discontinuous waterpassage (SBR) are earmarked by a relatively sophisticated controlsystems thus being excessively expensive for their use in low capacitysewage water sources.

DISCLOSURE OF INVENTION

The above stated disadvantages are overcome by a method and apparatusfor the sewage water treatment according to the present invention wheresewage water is supplied to the equalizing tank and from here drained tothe activation one: having been treated, the sewage water is supplied tothe sedimentitation tank and, after the sludge sedimentation, to thedrainage, The invention is based on an automatic interruption of theactivation process after a water volume drop in the equalizing tankbelow the specified minimum level, which is followed by a drainage ofthe excessive sludge from the activation tank. If the sewage water levelin the equalizing tank is increased in excess of its specified operatingvalue, the sludge re-pumping is terminated the activation process beingrestarted.

After an interruption of the activation process, it is advantageous todrain the excessive sludge after a properly adjusted time interval only.

The activation sewage water treatment plant as defined by the presentinvention is formed by an activation tank provided with an air supplyand with an overflow to the post-sedimentation tank. In addition thispost-sedimentation tank is provided with a pump for re-pumping thesludge from the post-sedimentation tank to the activation one and with adrainage outlet. Before the activation tank is arranged an equalizingtank provided with sewage water inlet as well as with a raw water pumptransporting sewage water from the equalizing tank to the activationtank, and with a float switch controlling the minimum and the operatingsewage water level in the equalizing tank. This float switch stops theactivation process and turns on successively the sludge pump when thesewage water drops under the minimum sewage water level and re-startsthe activation process and turns off the sludge pumpe when the sewagewater reaches the operating sewage water level.

The above described sewage water treatment plant is earmarked by aneffective connection of the design elements characterizing thecontinuous and the discontinuous water passage system for sewage watertreatment.

The advantages of the solution described by the present inventionconsist in the fact that, with a non-uniform sewage water inlet, theactivation and the post-sedimentation tanks are uniformly loaded thusenabling their dimensioning to an average daily sewage water inletvolume. In addition, this enables the use of a fine-bubble activationwhich is the most advantageous biological sewage water treatment methodfrom the viewpoint of the power consumption and the functional qualitiesto be used ever for the sewage water sources with minimum possiblecapacity values. The water treatment plant design described by thepresent invention enables additionally an increase in the activationtank sewage water level thus increasing the water volume containedherein. In this way, the necessary size of all the sewage -watertreatment plant can be substantially reduced.

An outstanding advantage of this arangement is represented by the factthat no skilled plant operators are required due to the automatic sludgedrainage from the activation tank as described by the present invention.

Another advantage is the substantial reduction of the daily air blowingtime caused by frequent blower function interruptions duringinsufficient sewage water inlet periods: this reduces the danger of thesludge autolysis that would otherwise be caused by the lack of nutrientsin the activation tank. During alternating re-pumping of cleared waterfrom the activation tank to the equalizing tank and vice versa, thenecessary nutrients are then supplied to the activation process from thesludge decomposing in the equalizing tank, This enables the operation ofthe whole water treatment plant for up to approximately 3 months withoutany sewage water inlet with no deterioration in the equipment treatmentability. This makes the sewage water treatment plant to be extremelysuitable for use in leisure objects characterized by an intermittentoperation in contradiction to other known types The suppression of thesewage water treatment plant activity after a longer interruption of thesewage water inflow can be supported by a higher setting of the minimumwater level in the equalizing tank pulling the float a bit higher thuscausing an increase in the re-pumping frequency and, in this way, areduction of the total blowing time per day. Another possibility isrepresented by an incorporation of a timing switch into the treatmentplant power supply turning the plant on only for a certain number ofhours per day.

An important contribution to the environment quality that cannot beneglected is the ability of the activation system described by thepresent invention to perform a sewage water denitrification including apartial removal of phosphorus in the biological way which was eitherimpossible or considerably complicated in small sewage water treatmentplants used hitherto. In this system, the denitrifying action isintroduced by interrupting the continuous activation process and by asuccessive re-pumping of the nitrified sewage water into an anoxical oran anaerobe equalizing tank environment. Successively, a mixture ofcleared (denitrified) and raw water is pumped into the activation tankfrom the equalizing one. In this way, the effectivity of the treatmentprocess is automatically increased by the system depending on the volumeof the sewage water inflow. During low inflow period, removal of organicimpurities and of nitrogen is provided by nitrification and successivedenitrification. With an increase in the sewage water inflow, the numberof re-pumping cycles is successively reduced thus lowering the degree ofdenitrification: a further reduction of the sewage water detention timein the activation tank leads to a successive reduction of thenitrification degree limiting lately the effectivity of the organicimpurities removal. With a reduced sewage water inflow, the effectivityof the treatment process is automatically increased up to waterdenitrification. In this way, the system responds as an entity to thesewage water inflow volume whereby the maximum plant passage is given bythe air raw water pump capacity (or by the capacity of another pumpused) being designed usually for two or three times as much as the dailyaverage sewage water volume. The most advantageous arrangement is theuse of an air-lift pump (the mamoth one) increasing continuously itscapacity with the increase in the water level in the equalizing tank andreducing it correspondingly with the decrease in the water level thusextending the total activation period duration till the systemswitchover.

The sewage water denitrification can be secured always by a properdimensioning of the water treatment plant tanks in order that minimumwater level in the equalizing tank can be reached more frequently duringthe plant operation thus increasing the frequency of the activationprocess interrupts.

BRIEF DESCRIPTION OF DRAWINGS

One of the possible implementations of the activation plant described bythe present invention is shown in the attached drawings.

FIG. 1 illustrates the ground plan of the water treatment plant,

FIG. 2 contains the vertical section drawing while the principal plantdiagram is shown in FIG. 3.

DESCRIPTION OF PREFERRED EMBODIMENT

The activation-type sewage water treatment plant as illustrated by FIG.1, 2 and FIG. 3 is formed by three functionally independent tanksconnected into a single system. They are the equalizing tank 1 withsewage water inlet 5, the activation tank 3 with an air supply, i.e.with the aerating pipe 7 connected to two aerating compressors 10, 11and with an overflow 19 to the post-sedimentation tank 4 and, finally,the post-sedimentation tank 4 with clear water drainage 6. Theequalizing tank 1 is provided with the raw water pump 13 driven by thecompressor 9: the pump is used to transport raw or pre-treated waterinto the activation tank 3. The equalizing tank 1 is further providedwith a screen 13a, which permits transfer of raw water to the pump 13.In addition, the plant comprises a water level measuring device, usuallya float switch 8 transforming the plant operating mode if the sewagewater level in the equalizing tank 1 drops below its minimum setting 15or exceeds its operation settin 16. The activation tank 3 is providedwith sludge pump 14 connected to the sludge pumping compressor 12 usedto re-pump the excessive sludge to the equalizing tank 1 up to thesludge level setting 2 in the activation tank 3. The post-sedimentationtank 4 is provided with the sludge recirculation pump 17 connected tothe sludge recirculation compressor 18: this pump is used to re-pump thesedimented sludge to the activation tank 3.

Sewage water is supplied to the equalizing tank 1 via the sewage watersupply 5. At the same time, the equalizing tank 1 is used to provide theprimary sedimentation and to store the excessive sludge re-pumped fromthe activation tank 3. With a standard operation level in the equalizingtank 1, the sewage water inflow is re-pumped into the activation tank 3.Having been treated in the activation tank 3, the mixture of clear waterand activation sludge is supplied to the post-sedimentation tank 4 thecleared water being drained by gravity from the post-sedimentation tank4 thus leaving the water treatment plant. The sedimented sludge ispermanently or intermittently re-pumped to the activation tank 3 by thesludge recirculation pump 17. A limited capacity of the raw water pump13 helps to reach the condition that even with a non-uniform sewagewater supply to the water treatment plant, the hydraulic load of theactivation tank 3 as wel as of the post-sedimentation one 4 is uniform.

During the operation of these small household sewage water treatmentplants with a non-uniform sewage water supply, the sewage water inflowe.g. at night is low enough so that the water level in the equalizingtank 1 drops below its minimum setting 15. This is the instant when theactivation process is disabled by the float swich 8, i.e. the aeratingcompressors 10, 11 are turned off together with the raw water pump 13thus puting the activation tank 3 out of operation. The sludgerecirculation pump 17 is turned off together with the aeration if theair source for the sludge pump 17 is a common compressor for aeratingand for the raw water pump 13 in water treatment plants with highercapacity values. If the sludge recirculation pump 17 is provided with anindependent compressor and if the inflow 19 from the activation tank 3to the post-sedimentation tank 4 is situated closely below the waterlevel in the activation tank 3, the sludge recirculation pump 17 is keptpermanently functioning.

At the same time, or better--after a specified time delay enabling thesedimentation of the activated sludge at the bottom of the activationtank 3, the sludge pump 14 is turned on starting to re-pump the contentsof the activation tank 3 into the equalizing tank 1. The inflow pipe ofthe sludge pumpe 14 is placed above the activation tank 3 bottom at thesludge level 2, i.e. In the height of the required sludge layer in theactivation tank 3 reached after the sedimentation time. As a rule, thesludge level 2, is set in such a way that the sludge after a 40 minutessedimentation takes up 1/4 up to 1/3 of the activation tank 3 volumewhich corresponds to a sludge concentration obtained by mixing ofapproximately 3 kg dry sludge in 1 m³ of the activation mixture for asupposed sludge index of ca 80. In this way, the sludge pump 14 removesonly the sludge in excess of the sludge level setting 2. The activationdelay of the sludge pump 14 is chosen so that the sludge is sedimentedat the bottom of the tank 3 before the pumping of the activation tank 3contents otherwise the necessary sludge quantity in the activation tank3 would be reduced due to pumping off the mixture of non-sedimentedsludge with water if the sludge re-pumping were too frequent. Clearwater is pumped off only having pumped off the excessive sludge to theinflow level of the sludge pump 14. As soon as the water level in theequalizing tank 1 reaches its operation setting 16 chosen always toexceed the minimum setting 15, the sludge pump 14 is turned off by thefloat switch 8 activating at the same time the aerating compressors 10,11, the raw water pump 13 and, possibly, the sludge recirculation pump17. The sewage water level in the equalizing tank 1 is increased to itsoperation setting 16 due to re-pumping a certain portion of theactivation tank 3 contents to the equalizing tank 1 or due to the sewagewater supply to the equalizing tank 1 or, possibly, due to a combinationof both above mentioned causes. The system then continues its activityas a standart sewage water treatment plant with continuous sewage waterpassage in the equalizing tank 1 below the minimum water level setting15.

The height of the minimum level setting 15 and that of the operationsetting 16 must be chosen with respect to the actual sewage water inflowvolume to the water treatment plant, to the amount of, oxygen dissolvedin the activation tank 3 depending on the capacity of the aeratingcompressors 10, 11, and due to the substance load of the sewage water.Another viewpoint may be the requirement concerning the cleared waterdenitrification.

The start delay of the sludge pump 14 depends on the sludgesedimentation speed and on the depth of the activation tank 3. Its usualvalues are between 30 and 90 minutes.

The water treatment plant with no requirements concerning the waterdenitrification is run with an activation tank 3 volume and with aneffective equalizing tank 1 volume usually equal to the daily averagevolume of the sewage water inflow to the water treatment plant. Theminimim level setting 15 is chosen so (usually 0,7 m above the bottom ofthe equalizing tank 1) that the disabling of the aerating and thefollowing sludge draw-off from the activation tank 3 occur approximatelyonce in a day up to once in a week. The difference between the minimumlevel 15 and its operation value 16 is set to a small value, usually notmore than 0,2 m in order to obtain the shortest possible activationinterrupt time.

The water treatment plant where water denitrification is required is runwith larger volumes of the equalizing tank 1 and of the activation tank3: these volumes are usually equal to twice as much as the daily averagewater inflow into the treatment plant. It is necessary to provide asufficient detention time of the sewage water in the activation tank 3to obtain complete nitrification with a sufficient volume of theequalizing tank 1 available at the same time between the minimum andmaximum levels 15 and 16 respectively to enable re-pumping of themaximum possible cleared water volume from the activation tank 3 to theanoxic environment of the equalizing tank 1 where this water is mixedwith raw water and then denitrified. The minimum level 15 is then set toobtain the aeration disabling, the sludge draw-off from the activationtank 3 and the cleared water-re-pumping to the equalizing tank 1 atleast once in a day.

I claim:
 1. A method of treating sewage water using suspended activatedsludge where sewage water is supplied to an equalizing tank and thenre-pumped to an activation tank, which in turn supplies apost-sedimentation tank following a clearing process, and is sent to adrain subsequent to a remaining sludge sedimentation, wherein the methodcomprises:interrupting a sludge activation process automatically when alevel of the sewage water in the equalizing tank drops below a minimumlevel setting; pumping off one of excess sludge and sludge with clearwater from the activation tank to the equalizing tank followinginterrupting the activation process; and stopping the pumping off andrestarting the activation process following the level of the sewagewater rising above an operational level setting.
 2. A method accordingto claim 1, wherein after interrupting the activation process, thepumping off occurs after a preset time delay.
 3. An apparatus fortreating sewage using a biological activating system with suspendedactivated sludge formed in an activation tank having an air supply andan overflow to a post-sedimentation tank provided with a first sludgepump for pumping sedimented sludge from a post-sedimentation tank to theactivation tank and a drainage outlet, the apparatus comprising:anequalizing tank in front of the activation tank having a sewage waterinlet; raw water pump means for transporting sewage water from theequalizing tank to the activation tank and a second sludge pump forpumping sludge from the activation tank to the equalizing tank; and afloat switch in the equalizing tank having a minimum sewage waterposition for disabling the activation process and starting the secondsludge pump and an operating water level position for stopping thesecond sludge pump and restarting the activation process.